Optical isolators are typically employed in bulk optical systems to eliminate one of two counter-propagating electro-magnetic light waves. An optical isolator is comparable with a diode having a low electrical resistance between its input pod and output pod and a very high resistance between the output port and input pod. Analogous, a light wave, fed via the optical input pod of an optical isolator to its output pod, is guided with low loss, and a counter-propagating light wave, i.e. a light wave being fed to the isolator's optical output port, is attenuated such that only a small amount thereof leaves the isolator via the input pod. Such an optical isolator has a unidirectional transmittance property, and cuts off most of the light fed back into its output pod.
A conventional optical isolator used in a pumiced laser cavity configuration is disclosed in the article "Single-Frequency Traveling-Wave Nd:YAG Laser", A. R. Clobes et al., Applied Physics Letters, Vol. 21, No. 6, September 1972, pp. 265-266. The bulk optical isolator illustrated and described in this article comprises a Faraday cell rotating the polarization of a light wave, depending on the light wave's propagation direction, when applying a magnetic field to it. In addition, a half-wave plate, being part of the optical isolator, is situated in the light path, such that a light wave passing through said Faraday cell prior to passing through the half-wave plate remains un-attenuated and a counter-propagating light wave is attenuated.
Other optical isolators, based on the same principle of affecting the light wave's polarization and guiding it through a polarization sensitive filter element, are listed below. These optical isolators have the advantage, with regard to electro-optic integration, that they are smaller, and some of them suited for integration into optical waveguides.
U.S. Patent, U.S. Pat. No. 3 830 555 with title "Non-reciprocal Waveguide Mode Conveder";
French Patent, FR-A 2 614 999 with title "Guide d'Onde Optique Bidimensionnel Monomode Ferrimagnetique, son Procede de Fabrication, et son Utilisation dans un Isolateur Optique Integre";
European Patent Application, EP-A 0 309 531 with title "Monolithic Monomode Waveguide Isolator and Application to a Semiconductor Laser";
German Patent, GE-A 3 741 455 with title "Optischer Isolator";
European Patent Application, EP-A 0 343 688 with title "Optical Element, Optical Disk and Rotary Encoder with the Optical Element";
European Patent Application, EP-A 0 397 089 with title "Light Isolator of Waveguide Type";
U.S. Patent, U.S. Pat. No. 4,973,119 with title "Optical Waveguide Isolator";
European Patent Application, EP-A 0,170 523 with title "Optical Polarization-State Conveding Apparatus for Use as Isolator, Modulator and the Like".
Most of these isolators, are complex, bulky active elements, employing the magneto-optic effect for rotation of the polarization of an electro-magnetic light wave, thus providing for a nonreciprocal transmittance properly. To affect the light wave's polarization, electro-magnetic materials, e.g. Gadolinium Gallium Garnet (GGG; Gd.sub.3 Ga.sub.5 O.sub.12), ferromagnetic garnet or Yttrium Iron Garnet (YIG; Y.sub.3 Fe.sub.5 O.sub.12), have to be employed. Additionally, electrodes, for applying an electrical field, have to be incorporated in these isolators.
Some of the disadvantages and problems of the above electro-magnetic optical isolators are discussed in the following. The electro-magnetic materials are hard to integrate with other optical devices. While a film of an electro-magnetic material itself can be grown by Liquid Phase Epitaxy (LPE) or sputtering, a film of suitable quality cannot be grown on a semiconductor substrate since their lattice constants and thermal expansion coefficients differ. Thus, it is difficult to integrate optical isolators, based on the electro-magnetic effect, with other optical devices.
There are magnetic semiconductor materials known in the art, e.g. CdMnTe, allowing the integration on conventional semiconductor substrates. A typical waveguide isolator, consisting of multiple layers of CdMnTe and CdTe grown on top of a semiconductor substrate, is disclosed in Japanese Patent Application JP-A 63 198 005 with title "Waveguide Type Isolator". The employment of magnetic semiconductor materials still results in bulky devices which are not easy to integrate with other devices.
From this point of view, it would be desirable to employ optical isolators, instead of the one's described above, made of semiconductor materials which can easily be grown on top or a substrate, f.e. consisting of semi-insulating GaAs or InP. This would allow a further reduction of size, resulting in higher integration densities, and simpler fabrication. The integration of such an optical isolator in an optical waveguide would be advantageous.
The present optical waveguide isolator employs directional waveguide couplers which are, as such, known in the ad. The smaller the size of these directional couplers is, the better they are suited for monolithic electro-optical integration (EOI).
The most commonly used directional waveguide coupler is hereinafter referred to as branching waveguide coupler. The simplest branching waveguide coupler is an Y-shaped waveguide, i.e. it consists of a waveguide stem at one of its ends being splitted into two branches. Depending on the refractive indices of the stem and each of said branches, their width the branching angle, and the embedding material, the branching waveguide coupler serves as power divider or mode splitter. Exemplary literature, relating to passive branching waveguide couplers, is listed below:
"Mode Conversion in Planar-Dielectric Separating Waveguides", W. K. Burns et al., IEEE Journal of Quantum Electronics, Vol. QE-11, No. 1, January 1975, pp. 32-39:
"Normalised Power Transmission in Single Mode Optical Branching Waveguides", H. Sasaki et al., Electronics Letters, Vol. 17, No. 3, February 1981, pp. 136-138;
"Operation Mechanism of the Single-Mode Optical-Waveguide Y Junction", M. Izutsu et al., Optics Letters, Vol. 7, No. 3, March 1982, pp. 136-138;
"Optical-Waveguide Hybrid Coupler", M. Izutsu et al., Optics Letters, Vol. 7, No. 11, November 1982, pp. 549-551:
U.S. Patent, U.S. Pat. No. 4,674,827 with title "Slab Type Optical Device";
U.S. Patent, U.S. Pat. No. 4,846,540 with title "Optical Waveguide Junction";
Most of these articles and patents relate to waveguide branches made of LiNbO.sub.3 or glass, only some of them mentioning semiconducting materials, such as GaAs, to be used. An important progress has been made towards waveguides consisting of semiconducting materials and being integrable on conventional semiconductor substrates, as described in the following section.
Examples for the monolithic integration of branching waveguide couplers and laser diodes are given in:
"Monolithic Integrated InGaAsP/InP Distributed Feedback Laser with Y-Branching Waveguide and a Monitoring Photodetector Grown by Metalorganic Chemical Vapor Deposition", K.-Y. Lieu et al., Applied Physics Letters, Vol. 54, No. 2, January 1989, pp. 114-116;
European Patent Application. EP-A 0 469 789 with title "Optical Branching Waveguide".